Normally, narrowband receiver systems have great difficulties in acquiring a desired carrier frequency in the presence of modulation, because of the proximity of the sideband frequencies to the carrier frequency. To facilitate acquisition of the carrier, most receivers scan throughout a specified band in search of the carrier frequency, the only frequency at which the receiver functions properly as a data/information demodulator. However, since the band generally contains many other frequency components which are produced during the modulation process at the transmit end, the receiver may mistake a sideband component for the carrier and lock onto it, resulting in, both, a termination of the carrier acquisition process and a lock to the wrong signal. While manual tuning to acquire the carrier (as opposed to automatic acquisition) may be adequate in some applications, for others it presents major difficulties, including the attention of well-trained personnel and cumbersome spectrum measurement equipment.
Several techniques designed to avoid lock onto undesired sideband signal components during automatic carrier acquisition are in use. Some "anti-sideband" circuits use a method by which the receiver scans through the frequency band, while sampling and storing information on specific spectral frequencies and the corresponding power levels. At the conclusion of the scan, a controller evaluates the stored values to determine the position of the carrier in accordance with characteristics of the spectral signature.
Another technique utilizes internal modulation by which the receiver recovers all detectable signal components. This information is used to modulate the incoming signal with an appropriate phase and level to assure suppression of the sideband components at the output of the circuit.
There are disadvantages associated with the above mentioned conventional carrier acquisition systems. Specifically, some of the anti-sideband methods require a high channel signal-to-noise ratio (S/N), severely limiting the performance when the communications path experiences signal loss due to fading. Other methods are slower in response and require more expensive circuitry. When internal modulation is used, noise components over a wide frequency band are also added during the process, strongly enhancing the overall noise power at the tuning control loop input. This noise substantially reduces the signal-to-noise ratio at the input of the loop, thereby limiting the S/N range over which the carrier may be detected. Furthermore, circuits which scan through the band and then process the data to identify the carrier signal, as described above, are usually awkward in design and require a programmed controller and a storage medium.